I have never heard of dialogs announcing pending updates, only those prompting you to update.

Perhaps you could post a screen shot of this.

Attached.

I have a similar problem. The message says it's updating snaps and I must close Thunderbird. It also says something like "13 days left" (the number varies). I close Thunderbird. In the morning I open it again and if I leave it open, the message will return. I'd just like to know what it means.

Yep, I still am not able to figure out why this is happening. It is strange and this is the first I have seen it.

Ok, SNAP is a package manager for Linux, and that is where I think this message is appearing from. What I think this message is trying to tell you is you need to restart Thunderbird to apply an already downloaded update and in X days the update will lapse.

That is all assumption, but it just sounds like the way Linux developers think. Force nothing and drop the subject if you ignore it. Even if it does leave you at significant risk by doing nothing.

I did a little googling and found https://www.reddit.com/r/Ubuntu/comments/vb6qxh/new_snap_update_notifications/

If anyone actually knows how this stuff works and can shed more light, perhaps they might chime in. There have been a number of similar posts in this forum of late, apparently the snap people do not do communications well if that is the source. But I really would like confirmation.

OK, I closed Thunderbird and did a system update. Rebooted and started Thunderbird again. This morning I seen the message again. so that didn't stop it.